The axial flow combines disclosed in U.S. Pat. Nos. 4,007,744; 4,103,691; 4,108,150; and 4,154,250 include a grain accelerator precleaner having a pair of toothed accelerator rolls which grip and accelerate threshed material downwardly across a horizontally directed air stream. In some operating conditions, it has been found that an excessive portion of the stream of cleaning air passes into the threshing area of the combine by way of the accelerator precleaner resulting in excessive discharge of dirty air from the front of the combine in the area of the operator's cab. This decreases the operator's visibility and increases the load on the air conditioner filter. The primary object of the present invention is to minimize the flow of the cleaning air to the threshing area of the combine by way of the accelerator rolls without reducing the flow of cleaning air to the cleaning area of the combine.